[wildwestpat]

Hi Erik

Looks as if you have found a good source of second use bits for your CNC adventures.

1. Skate type ball bearings can be used to make very good linear motions. There are some sophisticated commercial offerings that are right up there with the best. For low cost home brews the limits are the accuracy of the track. Your options on available metal for the track-way will determine the actual construction from which you can then design the trolleys for the gantry. There are ball races that have a grove or flange on the outer part and these are useful in reducing the complexity in some circumstances. Some even have 'v' groves and are designed to run on a 90 degree edge. You might get lucky and find some of these.

I would stay away from round tube as the trolley construction will get complicated compared with square or channel sections – this is a very personal choice and available materials will rule!

The use of the slippery plastic sold for making routing jigs might make a very cheap and easily built gantry leg support bearing. I think the 'Legacy Mill' link I sent you uses this approach. The bearing surface appear to be so close together that a simple slider would appear to be used and not ball bearings. I have used one of the fancy low friction materials sold for routing jigs to mke a sledge to enable me to machine cross grain wood that kept grabbing the the router table and fence. The home made sledge used to carry the wood that was being difficult slid like a dream over the cast alloy as if it was on well oiled ball bearings.

2. The racking of the gantry can be reduced by a number of different methods. Obviously making the gantry bearing surfaces long in the direction of travel helps and also increases rigidity. Long (wide) gantry supports will have less tendency to twist and using a drive to each side that is synchronised is the most common one used. Racking can also be sorted by using a figure of eight cord that is made of non stretchable material to link the two gantry legs. I am looking for a length of Kevlar cord with which to experiment. The problem is that there would be some 15 feet of cord linking the sides. (This sort of mechanism was used on large drafting tables when drawings were done full size using pencils! The larger boards were the size of the bed of the machine you are constructing. The resulting drawings were certainly accurate to 1/64 inch.)

You might find some of these old drawing tables being scrapped in which case you might get a couple of top mounted slides that could be adapted very easily for gantry leg supports. These used bar that was ground with a chrome finish and can be 1 inch or more in diameter. The rail being supported with cast brackets or channel throughout its length. A carriage containing ball races traps the carriage and maintain accuracy throughout the 5 foot or more travel..I you mention this as I got one that I used to make a superior circular saw sledge type of carriage for a friend when I was in Ireland. He tells me it is still working well and I am not having it back.

In my book a pile of useful bits is a prerequisite for making things and fuel the imagination for the next project ad infinitum.

Now looking forward to your build.

Regards

Pat